1. Field of the Invention
This invention generally relates to communication networks and protocols, with particular relevance to wireless networks, or other networks requiring minimal turnaround signaling time.
2. Discussion of the Related Art
Currently, communication networks are formed by interconnecting devices by wire or cable, and having each device conform to a protocol for sending messages along these wires and cables. In some instances, a portion of such a network may be implemented as a wireless connection, employing radio or infrared frequency signals between nodes. Such wireless connections are point-to-point, having a single communications device at each end, each tuned to each other at a frequency different from other devices in the same geographic area.
A wireless network, on the other hand, is formed without physical connections among the devices, employing, for example, radio frequency signals. Each device on the network is tuned to the same frequency, and each device conforms to a protocol for sending messages at this common frequency. The protocol may allow communication among all the devices in the network or the protocol may constrain each device to only communicate with a master device. Wireless networks offer a significant logistical advantage over wired networks, by obviating the need to run wires or cables to each device.
With increased availability of multimedia technologies, and the increased demand for information access, the market potential for residence or business based Local Area Networks (LANs) is growing. The ease of installation and expansion of a wireless network is certain to create a large demand for wireless LANs. For example, a central base station may provide wireless services, including voice, video, and data, to all the communications devices in one's home, or a wireless base station may provide for the communication among all the portable computers in an office, or all the computers on a campus. To be successful, however, the techniques and protocols employed in these wireless networks must not be significantly inferior to their wired network equivalents.
During the past decades, protocols have been developed for effectively and efficiently managing the transfer of information within networks of communicating equipment. An underlying premise in the development of these network protocols has been that of a wired network infrastructure. In a wireless network, the assumptions upon which the wired network protocols were developed may no longer be valid. Although most of the existing protocols are functionally extensible to wireless networks, their effectiveness and efficiency may be adversely affected by the lack of a direct connection among devices.
A common protocol employed for data communications in a wired network is a bus structure with a "broadcast" protocol. Devices on the bus monitor the bus, wait for a quiet period, then transmit. Collisions occur when a second device, having also waited for the quiet period, simultaneously begins to transmit. The broadcast protocol typically calls for the devices to cease transmission in the event of a collision, and try again at the next quiet period. Repeated collisions are avoided by having the devices each randomly change their time of response from the start of the quiet period, so that they will no longer respond simultaneously. This broadcast protocol, as the name implies, has its roots in radio transmission, and is still widely used for voice wireless networks, such as CB radios.
The broadcast protocol, however, is unsuitable for high speed data communications on a wireless network because collision detection on a wireless network is time-consuming. On a wired network, the protocol typically calls for an active assertion of one logic level, but the passive assertion (i.e. a non-assertion of the active level) of the other level. Collisions are detectable at the transmitter by monitoring the bus during the transmission of a passive level. If an active level is detected during this transmitter's transmission of a passive level, it necessarily implies a collision. The wired transmitter can automatically retransmit the message at the next quiet period. A device transmitting on a wireless network, however, is unable to detect whether another device is transmitting at the same time. The device transmitting, if it monitors the transmission frequency, will only detect its own transmission, because its power level will be significantly higher than that of a remote transmitter. The intended receiver, however, being remote from both transmitters, typically receives a garbled message caused by the simultaneous transmission by two transmitters on the same frequency. Because collisions are likely to occur, and the transmitter has no means to detect these collisions, the wireless broadcast protocol typically requires the intended receiver to acknowledge (ACK) the receipt of each message. If it doesn't receive the message, or receives a garbled message, it doesn't transmit the acknowledgement, or transmits a Not-acknowledged (NAK) signal. If the transmitter fails to receive an acknowledgement, it retransmits the prior message. The requirement for an acknowledgement from the receiver for each message in a wireless network has a compounding adverse effect, because the transmission of each acknowledgement can also cause collisions. As traffic density increases, the likelihood of collision increases exponentially because of the increased acknowledgement traffic, as well as the repeated transmissions with each collision.
Polling network protocols, wherein a master device polls each of the other devices for messages, are applicable to wireless networks. Such protocols, however, are inherently inefficient for networks with uneven traffic patterns. During the polling process, each device on the network is queried, and the polling of inactive devices consumes time. Most polling protocols allow for the suspension of the polling of a device after some period of inactivity, to save time, but such protocols must also include a means for the unpolled device to notify the master device when it becomes active again. Often this reactivation notification is accomplished by providing an auxiliary connection to the master device, for example an interrupt line common to all devices. The equivalent of an additional auxiliary connection in a wired network is an additional auxiliary frequency in a wireless network. Alternatively, a period of time can be set aside in each message period for a notification signal. The occurrence of a reactivation notification on this common line, or during the notification period, causes the master device to repoll all the devices on the network to determine which device is now active.
Thus, it is seen that the transformation of a wired network protocol to a wireless network protocol typically requires additional time, or frequency, or both. This added demand of time or frequency is for the transfer of control information for the management of the wireless network. It is the purpose of this invention to minimize the time required to communicate such control information within a network. Although the invention presented is particularly applicable to wireless networks, the principles embodied are equally applicable to minimize the time required to transfer control information on a wired network as well.